A conventional PTC thermistor is described below.
A conventional PTC thermistor is disclosed, for example, in Japanese Laid-open Patent No. 61-10203, in which a plurality of conductive sheets composed of polymer having PTC characteristic, and an inner-layer electrode and an outer-layer electrode composed of metallic foil are alternately laminated, and a side-face electrode layer is disposed at a facing side as a lead-out part.
FIG. 7 is a sectional view of a conventional PTC thermistor.
In FIG. 7, reference numeral is a conductive sheet having carbon black or other conductive particles mixed in a crosslinked polyethylene or other polymer material. Reference numeral 2 is a metallic foil of copper, nickel or the like, having openings 3 disposed at the start end and terminal end of the conductive sheet 1 and crimped alternately, and disposed at upper and lower sides of the conductive sheet 1, and an inner-layer electrode 2a and outer-layer electrode 2b composed of this metallic foil 2 and the conductive sheet 1 are laminated alternately to form a laminated body. Reference numeral 5 is a side-face electrode layer disposed to be connected electrically with an end of the inner-layer electrode 2a and outer-layer electrode 2b at the side facing the laminated body 4.
In thus constituted conventional PTC thermistor, its manufacturing method is described below.
First, carbon black or other conductive particles are mixed in polyethylene, and a rectangular conductive sheet 1 is formed, and an inner-layer electrode 2a and an outer-layer electrode 2b composed of a metallic foil made of copper or nickel, of which side is shorter than at least one side of the sides of the conductive sheet 1 by 0.5 to 3.0 mm, are laminated, so that one end is alternately aligned with one end of the conductive sheet 1 and that an opening 3 may be formed at other end, so that a laminated body 4 is formed. At this time, the uppermost side and lowermost side of the laminated body 4 are formed so that the outer-layer electrode 2b composed of metallic foil may be laminated.
Next, while heating the laminated body 4 to a temperature of 100 to 200 deg. C., it is compressed from above and beneath, the conductive sheet 1 is softened, and the conductive sheet 1 of the laminated body 4 and the inner-layer electrode 2a and outer-layer electrode 2b made of metallic foil are fixed.
Finally, at the facing side of the laminated body 4 fixed in the preceding step, a conductive paste is applied to connect electrically with an end of the inner-layer electrode 2a and outer-layer electrode 2b composed of metallic foil 2, and a side-face electrode 5 is formed, and then by crosslinking, a PTC thermistor is manufactured.
In such conventional PTC thermistor constitution, however, in order to lower the initial resistance value, the conductive sheet 1 and the inner-layer electrode 2a and outer-layer electrode 2b composed of metallic foil are laminated alternately and compressed thermally, but since they are made of different materials, when exposed to thermal impulse, peeling may occur between the conductive sheet 1 and the inner electrode layer 2a and outer electrode layer 2b made of metallic foil due to large difference in coefficient of thermal expansion, thereby increasing the resistance value. Further, in a conventional PTC thermistor as shown in FIG. 7, when bonding a PTC thermistor to a printed board by soldering, the solder is not bonded sufficiently to the side-face electrode 5 or outer-layer electrode 2b. As a result, defective connection occurs, and the soldered portion may be cracked by thermal impulse of high temperature and low temperature.
It is hence an object of the invention to present a PTC thermistor excellent in contact between the conductive sheet and inner-layer electrode and outer-layer electrode composed of metallic foil, and not increasing in the resistance value due to thermal impulse.